Bam Margera -- Fist Fight with Undefeated PRO BOXER ... Over a Chick

Bam Margera got into a full on street fight in Australia this week with an UNDEFEATED PRO BOXER ... and TMZ has learned it was all over a girl.

Sources close to the "Jackass" star tell us ... Bam and a close lady friend were hanging out in Adelaide, Australia ... when the two were confronted by the chick's ex-BF ... who just happens to be an undefeated featherweight boxing phenom.

We're told the two men exchanged words ... and then fists began to fly. Sources tell us ... Bam "held his own" ... and "didn't get hurt that bad."